Base station, mobile station, communication system, transmission method and reordering method

ABSTRACT

A radio communication system, that includes a source base station; a target base station; and a mobile station for receiving either first sequence number or second sequence number, or both of the first and second sequence numbers from the target base station, whereby the first sequence number is transferred from the source base station to the target base station and the second sequence number is added to PDCP (Packet Data Convergence Protocol) SDU (Service Data Unit) which is transferred from the source base station to the target base station.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of U.S. application Ser. No. 12/603,125,filed on Oct. 21, 2009, now pending, which is a continuation of PCTApplication of PCT/JP2007/059037, filed on Apr. 26, 2007, the contentsof each are herein wholly incorporated by reference. The presentapplication also relates to U.S. application Ser. No. 13/430,207, filedMar. 26, 2012 and U.S. application Ser. No. 13/894,062, filed May 14,2013.

BACKGROUND OF THE INVENTION

The present invention relates to a base station, mobile station,communication system, transmission method and reordering method.

Currently, for mobile communication systems such as mobile telephones,third-generation type service using CDMA has begun, however, research ofa next generation mobile communication system (LTE: Long Term Evolution)in which even faster communication is possible is being advanced by the3GPP (3rd Generation Partnership Project) (refer to 3GPP, “Requirementsfor Evolved UTRA (E-UTRA) and Evolved UTRAN (E-UTAN),” TR25.913 V7.3.0,Release 7, March 2006). Some of the large challenges in this researchinclude increasing the transmission rate and reducing transmissiondelay.

In the LTE communication system, in order to increase the transmissionrate and reduce transmission delay, the handover method used is designedto be at a higher level than that of a conventional system. In a mobilecommunication system, when a mobile station moves during communication,the base station that the mobile station is communicating with isswitched (handover) according to the reception status. Therefore, inorder to perform communication at increased transmission speed and withlow transmission delay, improving the level of the handover isessential. In a LTE communication system, a packet exchange system isbasic, so handover is a hard handover. In a hard handover, after theline connection with the base station that the mobile stationcommunicates with before moving is cut, a line is connected between themobile station and a target base station. In a hard handover, byobtaining system information about the target base station immediatelybefore performing the handover, it is possible to perform the handoverin a short time; however, transmission of user data becomes interruptedduring the handover.

Therefore, in order to reduce transmission delay it is important thatthe state of interrupted transmission be shortened and that the loss ofpackets while transmission is interrupted be prevented. In the case thatpackets do become lost while transmission is interrupted, the lostpackets are recovered in end-to-end retransmission of the packets, sotransmission delay becomes large.

Therefore, in a handover in a LTE communication system, a method isspecified in which among data that includes control information andpackets for the mobile station, the transmission of packets that havenot yet been transmitted to the mobile station from the source basestation is taken over by forwarding those packets to the target basestation from the source base station (refer to 3GPP, “Evolved UniversalTerrestrial Radio Access (E-UTRA) and Evolved Universal TerrestrialRadio Access Network (E-UTRAN),” TS36.300, Release 8, V8.0.0, April2007).

Takeover During a Handover

FIG. 21 is a drawing explaining takeover during a handover. In (A) ofFIG. 21, two base stations 1 a and 1 b are connected to a host station(for example a MME/SAE gateway) 2. A mobile station 4 exists within thecell 3 a of the base station 1 a, and is currently communicating withthe base station 1 a. In this state, as shown in (B) of FIG. 21, whenthe mobile station 4 moves in the direction toward the base station 1 band enters into the cell 3 b, a handover is executed and the basestation with which the mobile station 4 communicates is switched fromthe base station 1 a to the base station 1 b. Here, a base station thatis in communication with a mobile station before the mobile stationmoves is called the source base station, and the base station thatcommunicates with the mobile station after the mobile station moves iscalled the target base station.

The source base station 1 a stores packets that were received from thehost station 2 in an internal buffer, and sequentially sends the packetsthat are stored in that buffer to the mobile station 4. Therefore, whena handover occurs, the packets that are not sent to the mobile stationare stored and exist in the buffer. In (B) of FIG. 21, it is necessarythat before a handover, the packets n−2 to n be received and stored inthe buffer without being sent to the mobile station, and that after thehandover it is necessary that these packets be sent to the mobilestation 4 from the target base station. Therefore, when executing thehandover sequence, the source base station 1 a transfers (forwards) thepackets n−2 to n to the target base station 1 b. By using thisforwarding method, the target base station sends those packets to themobile station 4 immediately after the handover, so no interruption inthe packets occurs. Therefore, it becomes possible to execute high-speedhandover without performing end-to-end retransmission of the packets. Inthe explanation above, n−2 to n are numbers (sequence numbers) thatindicate the order of the packets.

Handover

FIG. 22 is a drawing explaining a handover in a LTE communicationsystem, and FIG. 23 is a drawing explaining the handover procedure thatis currently presumed for a LTE communication system.

Using a Measurement Report (report of the reception status of the basestation 1 and other surrounding base stations), the mobile station 4notifies the source base station 1 that a handover HO (Handover) isnecessary (1. Measurement Control).

The source base station 1 decides a target base station 1 b according tothe contents of the Measurement Report (2. HO Decision), and sends ahandover request (3. Handover Request) to that target base station 1 b.At that time, the source base station 1 a also sends information aboutthe mobile station (mobile station ID, QoS (Quality of Service)information, etc.). The target base station 1 b performs call-receivingcontrol based on that information (4. Call-Receiving Control).

After allowing the acceptance of the mobile station, the target basestation 1 b returns a handover response to the source base station (5.HO Response). After that, the source base station 1 a sends a handoverinstruction to the mobile station 4 (6. HO Instruction), thenimmediately afterwards starts taking over the data (packets) (Packettransfer: Forwarding).

The mobile station 4 receives the handover instruction, and then obtainssynchronization using L1/L2 signaling (7. Synchronization), and aftersynchronization has been obtained, sends a handover complete report tothe target base station 1 b (8. HO Complete). After this, the targetbase station 1 b sends a handover complete report to the host station 2(9. HO Complete). After receiving the handover complete report, the hoststation 2 changes the packet transmission path from the source basestation 1 a to the target base station 1 b (10. Path Change), andreturns a HO complete response to the target base station 1 b (11. HOComplete Response). The target base station 1 b uses a HO completeresponse to notify the source base station 1 a that the handover HO iscomplete (12. Resource Release). After that, the path between the sourcebase station 1 a and the host station 2 is eliminated (13. ResourceRelease).

Packet Order Alignment Control

When packets are forwarded (forwarded) during execution of the handoversequence described above, there is a possibility that packets that aretransferred by the target base station 1 b will be jumped over by thepackets that flow from the host station 2 resulting in the sequencenumbers becoming mixed up. When packets are transferred from the targetbase station 1 b to the mobile station 4 as are with the sequencenumbers mixed up, the mobile station 4 is unable to receive the packetsin the correct order, so the quality of communication is degraded, andas a result high-quality communication cannot be achieved just beforeand after a handover.

Therefore, in the LTE communication system, by using a method asdescribed below, the base station and mobile station are capable ofmaintaining packet order. FIG. 24 is a drawing explaining packet orderalignment wherein the target base station 1 b maintains the packet orderby sending packets that are transferred from the source base station 1 awith higher priority than packets that are received from the hoststation.

In FIG. 24, before a handover, packets n−5 to n are stored in the sourcebase station 1 a, and of these packets, packets n−5 to n−2 are sent tothe mobile station 4, however, packets n−1 and n are not sent to themobile station 4. In addition, of the packets that are sent to themobile station 4, packets n−5 and n−3 are not rightly received by themobile station 4 (NACK), and packets n−4 and n−2 are rightly received(ACK). Therefore, the mobile station 4 save the packets n−4 and n−2 in abuffer BF1, and does not save the packets n−5 and n−3.

When a handover occurs in this state, the source base station 1 atransfers (forwards) the packets n−5 and n−3, which were not rightlyreceived by the mobile station 4, and the packets n−1 to n, which havenot yet been sent to the mobile station 4, to the target base station 1b, and the target base station 1 b stores those packets in a buffer BF.Also, after the handover, the host station 2 sends two packets m to m+1that are intended for the mobile station 4 to the target base station 1b, and the target base station 1 b stores those packets in the bufferBF.

After communication with the mobile station 4 becomes possible, thetarget base station 1 b first sends the packets n−5, n−3 and n−1 to nthat were forwarded from the source base station 1 b to the mobilestation 4. Next, the target base station 1 b sends the packets m to m+1that were received from the host station 2. As shown in FIG. 25, themobile station 4 rearranges the sequence order of the packets n−4 andn−2 that were received before the handover, and the packets n−5, n−3,n−1 and m to M+1 that were received after the handover, and gives thosepackets in order to an upper layer.

In the explanation above, the case was explained in which all of thepackets n−5, n−3 and n−1 to n were transferred (forwarded) to the targetbase station 1 b, however, in some cases, only the packets n−5 and n−3will be transferred, and transferring of packets n−1 to n will bedelayed. FIG. 26 is a drawing explaining packet order arrangement inthat case. The target base station 1 b stores the transferred packetsn−5 and n−3 together with the packets m to m+1 that come from the hoststation 2 in a buffer BF, however, sends the packets n−5 and n−3 thatwere forwarded from the source base station 1 a to the mobile station 4first. After that, in the case that packets n−1 and n are forwarded fromthe source base station 1 a later, the target base station 1 b monitorswhether a set time (waiting time) has elapsed, and when the packets n−1and n still have not been forwarded from the source base station 1 aeven after the waiting time has elapsed, the target base station 1 bdetermines that forwarding has been completed and sends the packets mand m+1 that were received from the host station to the mobile stationto the mobile station. Even though the packets n−1 and n may be receivedfrom the source base station 1 a after forwarding has been completed,the target base station 1 b discards those packets.

The mobile station 4 executes a process for rearranging the order of thesequence numbers of the received packets (reordering). As shown in FIG.27, the mobile station 4 rearranges the sequence numbers of the packetsn−4 and n−2 that were received before the handover, and the packets n−5,n−3, m and m+1 that were received after the handover, and gives thosepackets in order to an upper layer.

Protocol Configuration

As described above, in a handover in a LTE communication system, packettransfer (forwarding) and packet reordering are necessary techniques.The relationship between these functions will be explained in moredetail here.

FIG. 28 is a drawing explaining the protocol configuration between amobile station and a base station. Between a mobile station and a basestation there is at least a PDCP (Packet Data Convergence Layer) layer,RLC (Radio Link Control) layer and a lower layer (MAC layer/physicallayer MAC/PHY). A packet routing function or the like is provided inMME/S-GW.

The main features of each protocol are as described below.

(1) PDCP: In the PDCP layer, the transmitting side compresses the upperprotocol header, as well as attaches a sequence number and performstransmission. The receiving side checks the sequence number, and bydoing so, performs a discarding process for redundant reception.Retransmission is not performed in the PDCP layer.

(2) RLC: The RLC layer is a layer having a retransmission function, andin the RLC layer, a sequence number is newly attached to the data thatis different from the sequence number that is attached to the data fromthe PDCP layer, and the data is then transmitted. For example, when datahaving a sequence number n is received from the PDCP layer, that data isdivided into a plurality of data, and to each division of data, sequencenumbers I(1), I(2), I(3), . . . are attached in the RLC layer, afterwhich the data is transmitted. The receiving side notifies the sendingside by using those sequence numbers I(•) to send a transmissionconfirmation (Ack/Nack signal) indicating whether the data were receivedrightly or improperly. When an Ack signal is returned, the sending sidedeletes the data that is saved, however, when a Nack signal is returned,the sending side retransmits the saved data.

(3) Lower Layer

-   -   MAC: The MAC layer is a layer that multiplexes/demultiplexes the        data of the RLC layer. In other words, the sending side        multiplexes the data of the RLC layer and transmits the data,        and the receiving side demultiplexes the received data of the        MAC layer to RLC layer data.    -   PHY: The PHY layer is a layer for transmitting and receiving        data by radio signals between the user terminal 4 and base        station 1, and converts MAC layer data to radio data, or        converts radio data to MAC layer data.

Data destined for a mobile station first flows from the upper layer (forexample IP layer) to the PDCP layer to become a PDCP SDU (Service DataUnit), then header information (PDCP layer sequence number, etc.) isattached to become a PDCP PDU (Protocol Data Unit).

The PDCP PDU is sent to the RLC to become a RLC SDU, where headerinformation (RLC layer sequence number, etc.) is further attached tobecome a RLC PDU. The RLC PDU passes through the lower layer processingafter which it arrives at the RLC layer of the mobile station. In thisRLC layer, the header is deleted and the RLC SDU is reassembled, then inthe PDCP layer, the PDCP PDU header is deleted to become a PDCP SDU,then the data is sent to the upper layer.

In this kind of protocol configuration, in a LTE communication system,the forwarding of packets is performed in PDCP SDU units, and reorderingis performed in PDCP PDU units. When forwarding is performed in PDCP SDUunits, header information such as a sequence number is not attached tothe PDCP SDU unit packet, so that sequence number is not forwarded.

Therefore, in a case where the forwarding is performed in PDCP SDUunits, it is necessary to forward PDCP SDU data and harder informationincluding the sequence number separately.

The RLC SDU data and PDCP PDU data are essentially the same data, so inthe specification for the present invention, unless specificallyspecified, these will simply be called packets, and when a packet numberis provided, that number is the sequence number of the PDCP PDU data.

Operation of the Source Base Station

FIG. 29 is a flowchart of the operation of a source base station duringa handover.

When the source base station 1 a receives the reception field strengthfrom the user terminal 4 by way of a Measurement Report (step 101), thesource base station 1 a determines whether or not a handover HO isnecessary (step 102), and when a handover is not necessary, returns tothe beginning.

However, when it is determined that a handover is necessary, the sourcebase station 1 a decides a target base station 1 b according to thecontents of the Measurement Report, and sends a handover request to thattarget base station 1 b (step 103).

After that, the source base station 1 a receives a handover responsethat is sent from the target base station 1 b (step 104), and forwardsthe remaining packets to a target base station (step 107). Afterreceiving a resource release message that is sent from the target basestation 1 b (step 108), the source base station 1 a executes theresource release (step 109).

Operation of the Target Base Station

FIG. 30 is a flowchart of the operation of a target base station duringa handover.

After receiving a HO request from the source base station 1 a (includingthe mobile station ID, QoS information, etc.) (step 121), the targetbase station 1 b performs call receiving control based on thatinformation, and determines whether to allow the acceptance of themobile station (step 122), and when the mobile station is not allowed,performs post processing (step 130) and ends handover control.

On the other hand, when acceptance of the mobile station is allowed, thetarget base station 1 b returns a HO response to the source base station1 a (step 123). The target base station 1 b then stores the packets thatare forwarded from the source base station 1 a in a buffer (step 124),and receives a HO complete report from the mobile station 4 (step 125).After receiving the HO complete report, the target base station 1 bsends a HO complete report to the host station 2 (step 126). The hoststation 2 receives the handover complete report, then changes the packettransmission path from the source base station 1 a to the target basestation 1 b, and returns a HO complete response to the target basestation 1 b. After receiving the HO complete response from the hoststation 2 (step 127), the target base station 1 b starts sending thepackets that were forwarded from the source base station 1 a to themobile station preferentially, and after those packets have been sent,sends the packets that were received from the host station 2 to themobile station (scheduling: step 128). In addition, at the same time asstep 128, the target base station 1 b sends a resource release to thesource base station 1 a (step 129), then performs post processing (step130) and ends handover control. In the scheduling process of step 128,when forwarding of packets from the source base station 1 a is delayed,the target base station 1 b monitors whether a set time (Waiting Time)has elapsed, and when no packets have been transferred even though theWaiting Time has elapsed, the target base station 1 b determines thatforwarding has finished and sends all the packets that were receivedfrom the host station 2, and even though packets may be received fromthe source base station 1 a after forwarding has finished, the targetbase station 1 b discards those packets.

Operation of a Mobile Station

FIG. 31 is a flowchart of the operation of a mobile station during ahandover.

The measurement unit of the mobile station 4 sends a notification of thereception field strength or the like to the source base station by wayof a Measurement Report (step 151). After that, the mobile station 4waits for a HO instruction from the source base station 1 a, and afterreceiving a HO instruction (step 152), obtains synchronization with thetarget base station 1 b using L1/L2 signaling (step 153), and aftersynchronization has been obtained, sends a handover complete report tothe target base station 1 b (step 154), then, in the case where packetsare received from the target base station 1 b, the mobile station 4executes a reordering process (steps 155 to 160).

In other words, when the control unit of the mobile station receiveslower layer packets from the target base station 1 b, the control unitcreates RLC SDU data and delivers that RLC SDU (PDCP PDU) data to areordering unit (step 155). The reordering unit checks whether there areany missing sequence numbers (step 156), and when there are no missingsequence numbers and the sequence numbers are continuous, delivers PDCPSDU data obtained by removing a header from the RLC SDU (PDCP PDU) datato the upper layer (step 160). However, when there is a missing sequencenumber, the control unit instructs the reordering unit to save the RLCSDU (PDCP PDU) data. By doing so, the reordering unit saves the PDCP PDUdata (step 157), and checks whether RLC SDU (PDCP PDU) data having acontinuous sequence number has been received (step 158). When RLC SDU(PDCP PDU) data having a continuous sequence number has been received,the reordering unit delivers PDCP SDU data obtained by removing a headerfrom the RLC SDU (PDCP PDU) data to the upper layer, a well as deliversPDCP PDU data obtained by removing a header from the saved PDCP PDU datato the upper layer (step 160).

However, in step 158, when RLC SDU (PDCP PDU) data having a continuoussequence number is not received, the reordering unit monitors whether apreset amount of time has elapsed (step 159), and when that set amountof time has not elapsed, repeats the process from step 157; however,when the set time has elapsed, the reordering unit delivers PDCP SDUdata obtained by removing a header from the saved PDCP PDU data to theupper layer even though the sequence numbers may not be continuous (step160).

Problems

In a LTE communication system, the following problems exist whenexecuting packet forwarding during a handover. That is, as was describedabove, when a handover is executed in a LTE communication system, aprocess for taking over the packet for the mobile station that remainsat the source base station 1 a is executed, and packets are forwarded toa target base station during that taking over process. However, in thehandover control described above, if the value of the waiting time bythe target base station 1 b is small, the target base station 1 b beginssending the packets received from the host station regardless of whetheror not all of the packets have been forwarded, so a problem occurs inthat the packets that have not yet been forwarded are discarded. On theother hand, if the value of the waiting time is large, there is aproblem in that the target base station 1 b cannot send the packetsreceived from the host station until the waiting time has elapsed eventhough all of the packets may have already been forwarded, so atransmission delay occurs. In other words, in conventional handovercontrol there is an increase in communication delays and throughput isdegraded, making it impossible to maintain high-quality communicationimmediately before and after a handover.

As first related art is a method of notifying the target base station ofthe last packet to be forwarded from the source base station (refer toSamsung, “Method to Release Resources at Source ENB During Handover,”R3-061032, RAN3#53, September 2006). When forwarding is delayed, thetarget base station that has been notified of the last packet cantransmit the packets received from the host station by starting thesequence numbers of those packets from the sequence number of the lastpacket+1. In addition, by comparing the sequence numbers of the packetsthat are forwarded with the sequence number of the last packet, it ispossible to forcibly end the waiting time and to detect the end offorwarding with optimum timing. However, in the case that the lastpacket is deleted during forwarding, the target base station will beunable to accurately detect the last packet.

Moreover, as second related art is a mobile communication system formaking high-speed packet data transfer possible with no data loss duringa handover between base stations during high-speed packet communication(refer to Japanese patent publication No. JP2004-282652A). When ahandover occurs in this mobile communication system, the source basestation of the handover transfers (forwards) packet data to the targetbase station of the handover. However, there is an increase incommunication delay due to reordering by the mobile station, and thereis no improvement in the degraded throughput.

Furthermore, as third related art is a method in which the target basestation jumps the packets sent from the host station without waiting forforwarded packets to arrive (refer to Japanese patent application No.2006-086537A). In this method, by distinguishing the packets that arereceived from the source base station from the packets that are receivedfrom the host station, transmission is possible in which the packetsjump. However, it is necessary for the mobile station to have two ordercontrol functions, and the control thereof becomes complicated.

SUMMARY OF THE INVENTION

Taking into consideration the aforementioned problems, it is the objectof the present invention to quickly transmit packets that are sent froma host station (for example, a device that is different from a sourcebase station, and that sends data (packets) to a target base station) toa target base station to a mobile station.

Another object of the present invention is to properly reorder forwardedpackets even when there is only one order-control function and even whenpackets that are forwarded from a source base station are mixed withpackets that are sent from the host station by excluding the packetsthat are sent from the host station as the object of order control.

Providing items that are described in the embodiments that are notdescribed in the related art can also be considered as another object ofthe present invention. Preferably, such items are necessary forobtaining advantages that cannot be obtained with the related art.

Reordering Method

The reordering method of the present invention is a reordering method ofsending data (for example, packets), to which information (for example,numbers) that indicates the order of the packets is added, from a targetbase station to a mobile station, and rearranging the data (packets) atthe mobile station according to this order information.

In the reordering method of the present invention, the target basestation correlates discriminating information which makes it possible todiscriminate data that is forwarded from a source base station from datathat is not obtained via the source base station, with data that is sentto the mobile station, and transmits the discrimination information withthe data from the target base station to the mobile station. Preferably,in performing the reordering process, the mobile station discriminatesthe data forwarded from the base station from the data that is notobtained via the source base station based on the discriminationinformation. Moreover, preferably, the data that is not obtained via thesource base station is data that is received from a gateway device.Furthermore, preferably, the data that is sent to the mobile station isin the form of packets.

The reordering method of the present invention, comprises: a step offorwarding packets, which has not yet been sent to the mobile station,or data, for which a confirmation response of right reception has notbeen received from the mobile station, to a target base station from asource base station; sending that data to the mobile station from thetarget base station, as well as sending data that is received from ahost station to the mobile station; a step of, in the case of sendingdata received from the host station with the order jumped, giving thedata information indicating that the data is jump data, then sending thedata from the target base station to the mobile station; and a step ofdeciding the jump data is not the object of reordering and performingthe reordering process on data that is the object of reordering.

Another reordering method of the present invention comprises: a step offorwarding data that has not yet been sent to the mobile station or datafor which a confirmation response of proper reception has not beenreceived from the mobile station to a target base station from a sourcebase station; a step of sending data that is forwarded from the sourcebase station before a first set period of time ends and data receivedfrom a host station from the target base station to the mobile station;a step of discarding data that is forwarded from the source base stationafter the first time period has ended; a step of adding information todata to specify the data is the last data that is object of reorderingfrom among data that was received before the first time period ends oradding the information to other data that is sent immediately after thatdata and sending the data from the target base station to the mobilestation and a step of performing reordering process until a second timeperiod ends, and when the data that contains the aforementionedinformation is received, ending the reordering process even though thesecond time period has not yet ended.

The reordering method described above, also preferably comprises: a stepof giving packets information indicating that the packets are jumppackets when sending packets to the mobile station that were receivedfrom a gateway with the order jumped and sending the packets to themobile station from the target base station; and a step of decidingthose jump packets are not objects of reordering, and performing thereordering process on packets that are the objects of reordering at themobile station.

Communication System

The present invention is a communication system in which data (forexample, packets), to which information that indicates the order of thedata is added, is sent from a target base station to a mobile station,and the data is rearranged in number order at the mobile station.

The communication system of the present invention comprises a mobilestation, a source base station that communicates with the mobile stationbefore a handover, and a target base station that communicates with themobile station after a handover; where (1) the source base stationcomprises: a buffer that saves data that was received from a hoststation; a data transmission unit that transmits the data that is savedin the buffer to the mobile station; and a control unit that forwardsdata that was not sent to the mobile station before execution of ahandover sequence, or data for which a confirmation response of rightreception has not been received from the mobile station to the targetbase station; (2) the target base station comprises: a buffer that savesdata that is received from the source base station during execution of ahandover sequence and data that is received from the host station; acontrol unit that performs control so that data that is received fromthe source base station is sent preferentially to the mobile station,and when sending data to the mobile station that is received from thehost station with the order jumped, adds information to the data toindicate that the data is jump data; and a transmission unit thattransmits data to the mobile station; and (3) the mobile stationcomprises: a buffer that saves data that is received from a basestation; and a reordering control unit that decides the jump data is notan object of reordering, and performs reordering on data that is theobject of reordering.

Another communication system of the present invention comprises: amobile station, a source base station that communicates with the mobilestation before a handover, and a target base station that communicateswith the mobile station after a handover; where (1) the source basestation comprises: a buffer that saves data that is received from a hoststation; a data transmission unit that transmits the data that is storedin the buffer to the mobile station; and a control unit that forwardsdata that is not sent to mobile station before execution of a handoversequence, or data for which a confirmation response of right receptionhas not been received from the mobile station, to the target basestation; (2) the target base station comprises: a buffer that saves datathat is received from the source base station during execution of ahandover sequence and data that is received from the host station; acontrol unit that performs control to send data that is forwarded fromthe source base station before a first set period of time ends to themobile station preferentially as the object of reordering, as well asadds information to data to indicated that the data is last datareceived as an object of reordering before the first set period of timeends, or adds the information to other data that is transmittedimmediately after that data and sends the data from the target basestation to the mobile station, and discards data that is forwarded fromthe source base station after the first set period of time has ended;and a transmission unit that transmits data to the mobile station; and(3) the mobile station comprises: a buffer that saves data that isreceived from the base station; and a reordering control unit thatperforms reordering until a second set period of time ends, and whendata that contains information identifying that data is the last datathat is an object of reordering, is received or when data that containsinformation indicating that the last data that is an object ofreordering has already been received is, is received, ends thereordering process even though the second set period of time has not yetended.

Base Station/Mobile Station

Another form of the present invention is a base station and a mobilestation that form a first or second communication system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing explaining a first embodiment of the invention.

FIG. 2 shows an example of the format of a PDCP PDU packet.

FIG. 3 is a drawing explaining packet processing in the PDCP layer andRLC layer before a handover sequence.

FIG. 4 is a drawing explaining packet processing in the PDCP layer andRLC layer during a handover sequence.

FIG. 5 is a drawing showing the construction of a base station.

FIG. 6 is a drawing showing the construction of a mobile station.

FIG. 7 is a flowchart of the operation of a target base station in afirst embodiment of the invention.

FIG. 8 is a flowchart of the operation of a source base station in afirst embodiment of the invention.

FIG. 9 is a flowchart of the operation of a mobile station in a firstembodiment of the invention.

FIG. 10 is a flowchart of the reordering process by a mobile station.

FIG. 11 is a drawing explaining a second embodiment of the invention.

FIG. 12 shows an example of the format of a PDCP PDU packet.

FIG. 13 is a drawing explaining packet processing in the PDCP layer andRLC layer during a handover (1/2).

FIG. 14 is a drawing explaining packet processing in the PDCP layer andRLC layer during a handover (2/2).

FIG. 15 is a flowchart of the operation of a target base station in asecond embodiment of the invention.

FIG. 16 is a flowchart of the operation of a mobile station in a secondembodiment of the invention.

FIG. 17 shows an example of attaching PDCP PDU sequence numbers SN toPDCP SDU data and performing notification via the data plane (U-plane).

FIG. 18 shows an example of attaching PDCP PDU sequence numbers SN toPDCP SDU data and performing notification via the U-plane, as well asperforming notification of sequence numbers via the C-plane.

FIG. 19 is a drawing showing the procedure of the handover sequence inFIG. 18.

FIG. 20 shows an example when there is absolutely no PDCP PDU data toforward.

FIG. 21 is a drawing explaining the takeover of communication during ahandover.

FIG. 22 is a drawing explaining a handover in a LTE communicationsystem.

FIG. 23 is a drawing explaining the handover procedure that is currentlypresumed for a LTE communication system.

FIG. 24 is a first drawing explaining the reordering process by a mobilestation.

FIG. 25 is a second drawing explaining the reordering process by amobile station.

FIG. 26 is a third drawing explaining the reordering process by a mobilestation.

FIG. 27 is a fourth drawing explaining the reordering process by amobile station.

FIG. 28 is a drawing explaining the protocol configuration between amobile station and a network.

FIG. 29 is a flowchart of the operation of a source base station duringa handover.

FIG. 30 is a flowchart of the operation of a target base station duringa handover.

FIG. 31 is a flowchart of a mobile station during a handover.

DESCRIPTION OF THE PREFERRED EMBODIMENTS (A) Theory of the PresentInvention

With the present invention, the problems described above are solved bymaking it possible for a base station and a mobile station to executethe two following procedures.

Procedure 1: When there is a delay in data forwarding from the sourcebase station after a handover, the target base station sends data thathas already been received from the host station without waiting toreceive the delayed data (jump transmission) and the mobile station ismade to be able to recognize that the data. In other words,discrimination information is included in, attached to or correlatedwith the data in order to recognize that the data is jump data, andtransmitted with the data or transmitted using a control channel.

Procedure 2: When it is detected that jump transmission has occurred,the mobile station decides the jump data is not the object of reorderingand holds it in a buffer, and the mobile station waits for the databeing forwarded from the source base station to arrive. In other words,the mobile station discriminate data that is forwarded from the sourcestation from data that is transmitted without going through the sourcebase station using the discrimination information and performsreordering of the data that is forwarded from the source station.

In the conventional method, when data forwarding from the source basestation to the target base station is delayed, the mobile station mustwait transmission of the data that is forwarded from the host station tothe target base station. Therefore, by waiting until a specified amountof time (Waiting Time) has elapsed for that data from the source basestation to arrive, a problem occurs in that delay in communicationincreases and the throughput is degraded. However, as described above,by performing jump transmission of data, it is possible to quickly senddata that was received from the host station to the mobile station evenwhen data transfer (forwarding) from the source base station is delayed,and in doing so, communication delay is reduced. Therefore, whencompared with the conventional method, the present invention is able tomaintain high-quality communication immediately before and after ahandover.

(B) First Embodiment

FIG. 1 is a drawing explaining a first embodiment of the invention, andin this embodiment, the case of adding order information to each packetis explained, however, it is also possible to use data having aspecified size.

Here it is presumed that before a handover, packets n−5 to n are storedin the source base station 11 a, and of these packets, packets n−5 ton−2 are transmitted to the mobile station 14, however, packets n−1 and nare not transmitted to the mobile station 14. For example, the packetsn−1 and n arrived at the source base station 11 a after the radiocommunication line between the source base station 11 a and mobilestation 14 was cut, so these packets n−1 and n could not be transmittedto the mobile station 14. Also, of the packets that were transmitted tothe mobile station 14, it is presumed that the mobile station 14 couldnot rightly receive the packets n−5 and n−3 (NACK), however was able torightly receive the packets n−4 and n−2 (ACK). Therefore, the mobilestation 14 saves packets n−4 and n−2, but does not save packets n−5 andn−3.

When a handover occurs in this state, the source base station 11 atransfers (forwards) the packets n−5 and n−3 that could not be rightlyreceived by the mobile station 14 and the unsent packets n−1 and n tothe target base station 11 b. The forwarding of the packets will beexplained below, however, the invention is not limited to this example.

In addition, after the handover, the host station 12 transmits twopackets m to m+1 that are destined for the mobile station 14 to thetarget base station 11 b. It is presumed that the transfer (forwarding)of packets n−5 to n is delayed.

When that the target base station 11 b receives packets m and m+1 fromthe host station 12 before packets n−5, n−3, and n−1 to n are forwardedfrom the source base station 11 b, the target base station 11 b addsjump ID code F to the packets m and m+1 that are received from the hoststation 12 and sends those packets to the mobile station 14 first (jumptransmission).

The mobile station 14 saves the packets that were received from the basestation and to which jump ID code F is added in a buffer BF2, andexcludes those packets as objects of the reordering process. In (A) ofFIG. 1, the state is shown in which the mobile station 14 saves packetsm and m+1 in the buffer BF2, and saves the packets n−4 and n−2 that werereceived before the handover in buffer BF1.

Next, the target base station 11 b transmits the packets n−5, n−3 andn−1 to n that were forwarded from the source base station 11 a to themobile station 14. The mobile station 14 saves the packets n−5, n−3 andn−1 to n that were received from the target base station 11 b in thebuffer BF1, then executes the reordering process for these packets thatwere received after the handover and the packets n−4 and n−2 that werereceived before the handover (see (B) of FIG. 1), and delivers thepackets to the upper layer in the order of continuous sequence numbers.

In a case where the mobile station 14 do not receive packets havingcontinuous sequence number even when a prescribed time has been elapsed,the mobile station ends reordering process, and rearranges the order ofthe packets that have been already received and delivers them to theupper layer.

The mobile station 14 then delivers the packets to which the jump IDcode F has been added in order to the upper layer.

During forwarding, a source base station can transfer an entire packetto a target base station, or can transfer data of just part of thepacket (user data portion). Preferably, order information is added tothe forwarded data.

In addition, in FIG. 1, sequence numbers (order information) m, m+1 areassigned to the packet for which jump transmission is performed,however, the target base station 11 b can assign arbitrary sequencenumbers, can add numbers that overlap the sequence numbers that areadded to the forwarded packets, or can add numbers that do not overlap.

Jump ID Code

As an example of jump ID code F that is added to packets, a 3-bit ‘type’field that is included in the PDCP PDU header is used. In other words,in that type field, a new type number is defined as jump ID code F, andthat type number is added to a packet for which jump transmission isperformed. FIG. 2 is an example of the PDCP PDU format, where (A) is anexample of format when there is no header, (B) is an example of formatwhen the PDCP PDU sequence number is not added, and (C) and (D) areexamples of format when the PDCP PDU sequence number is added. In theformat of (C) and (D), a type field and PID field are defined in theheader HD, where the type field indicates the PDCP PDU type. The PIDfield is a field that indicates the header compression type that is usedfor the data included in the data portion. In the type field, ‘type=000’and ‘type=001’ are already regulated, however, the types ‘type=010 to111’ are not regulated and are unused. Therefore, ‘type=010’ is used asthe type number (jump ID code) for discriminating the PDCP PDU for whichjump transmission is performed.

PDCP Layer and RLC Layer Processing Before and after Handover Control

FIG. 3 is a drawing explaining the packet processing of the PDCP layerand RLC layer before a handover. The source base station 11 b storespackets n−5 to n−2 of the PDCP layer in a buffer ((A) of FIG. 3), and inthe RLC layer the packets are divided into a plurality of data as shownin (B), then RLC layer sequence numbers I, I+1, I+2, . . . , I+6 areadded to the divided data, and the data (RLC PDU data) are sent to themobile station 14. Before a handover, the mobile station performs ordercontrol for the PDCP layer using RLC SDU(PDCP PDU) and PLC PDU in theRLC layer. In (C), (D) of FIG. 3, the mobile station 14 did not rightlyreceive the divided data I, I+4 and I+5, or in other words, packets n−5and n−3, however, the mobile station 14 did rightly receive packets n−4and n−2. Before a handover, the mobile station 14 performs order controlfor the PDCP layer using RLC SDU (PDCP PDU) and PLC PDU in the RLClayer, however, after a handover, the RLC layer is regulated to beinitialized. Therefore, processing for order control moves to the PDCPlayer, and the mobile station 14 executes the handover sequence in thePDCP layer.

During reordering, the mobile station 14 starts a timer in order todetermine when reordering ends.

The arrival of the packets m and m+1 that are received from the hoststation 2 is early, so jump ID code is attached to those packets m, m+1and they are sent to the mobile station 14 first (jump transmission).The mobile station 14 saves the packets that are received from the basestation and to which jump ID code is attached in a buffer B2 (see FIG.1), and removes them as objects of the reordering process. After that,the mobile station 14 executes the reordering process for the packetsn−5, n−3, and n−1 to n that were received from the target base station11 b and the packets n−4 and n−2 that were received before the handover,then delivers the data to the upper layer.

When the specified time TM of the timer has elapsed, the mobile station14 ends the reordering process and sends the packets that have beenreceived to the upper layer even though there may be missing packets.

Window Control

FIG. 4 is a drawing explaining the packet processing of the PDCP layerand RLC layer after a handover, and shows in detail window control.

In consideration of the fact that more than the allowable amount of dataarrives, the mobile station 14 internally generates a buffer sizewindow, and during the reordering process, performs the following windowcontrol. In the example shown in FIG. 4, the left end of the window WDis initially the number n−5 of the expected data, and the right end ofthe window is the number decided according to the amount of allowabledata (window buffer size), and is taken to be n−2. The mobile station 14does not apply the window process to the packets to which jump ID code Fhas been added.

In the initial state (window state (0)), when the mobile station 14receives the expected packet (n−5) and executes the reordering process,the window state becomes as shown in (1). Therefore, the mobile station14 delivers the expected packet (n−5) and the packet (n−4) thatcontinues after the packets(n−5) to the upper layer. Thereafter, thewindow state becomes as shown in (2), and in this state, when the mobilestation 14 receives the expected packet (n−3) and performs thereordering process, the window state becomes as shown in (3). Therefore,the mobile station 14 delivers the expected packet (n−3) and the packet(n−2) that continues after the packet(n−3) to the upper layer. Thewindow state then becomes as shown in (4), and in this state, the mobilestation 14 waits to receive the packets (n−1) and n, and when thepackets are received within the set period of time T_(M), the mobilestation 14 then delivers the packets to the upper layer. However, whenthe mobile station 14 does not receive the packets (n−1) and n withinthe set period of time T_(M), the mobile station 14 delivers the packetsstarting from packet m that are saved in the buffer BF2 to the upperlayer, and moves to normal control as before handover. As was explainedabove, by removing the jump packets from the reordering process andperforming window control as described above, it is possible to executethe reordering processing on just packets n−5 to n.

In this window control, when a packet is received that has a sequencenumber that is smaller than the sequence number on the left end of thewindow, the mobile station 14 deletes that packet. Also, when a packetis received that has a sequence number that is larger than the sequencenumber on the right side of the window, the mobile station 14 takes thesequence number of that packet to be the sequence number on the rightside of the window and changes the sequence number on the left side ofthe window according to the window buffer size, while at the same time,delivers the packets that have moved out of the window range and thathave been received to the upper layer.

If this window processing is applied to the jump packets as well, at theinstant that packet m is received, the window state will be such thatthe left side of the window is n−2 and the right side of the window ism. In this case, the mobile station 14 gives up on receiving the packetsn−5 and n−3 that have not yet been received, and delivers the alreadyreceived packets n−4 and n−2 to the upper layer. In addition, the mobilestation decides the sequence number of the right side of the window tom, and decides the sequence number of the left side of the window to thenumber n−1 that is determined according to the window size, then afterthat waits for packet n−1, packet n, packet n+1, . . . , packet m−1 toarrive. However, since packet n+1 to packet m−1 are packets that do notactually exist, the mobile station uselessly waits to receive thosepackets, and trouble occurs in packet processing.

Construction of a Base Station

FIG. 5 is a drawing that shows the construction of a base station, andshows the buffer unit, scheduler unit, transmission/reception unit andcontrol unit.

The buffer unit 21 is memory for storing packets that come from the hoststation, and packets that are forwarded from an adjacent base station(source base station). In the FIG. 5, two buffers 21 a, 21 b arephysically provided, however, construction is also possible in whichjust one memory is physically provided, and that one memory is used bydividing it using software.

The scheduler unit 22 selects a mobile station from among a plurality ofcommunicating mobile stations with which to perform radio transmission,fetches packets for that mobile station that are stored in the bufferunit and sends them to the transmission/reception unit 23. Thetransmission/reception unit 23 encodes and modulates the packets thatare input from the scheduler unit 22, and transmits the actual datausing radio communication. In addition, the transmission/reception unit23 receives and demodulates control signals and various data that aresent from the mobile station.

The control unit 24 comprises a buffer management unit 24 a, HO controlunit 24 b and measurement control unit 24 c. The buffer management unit24 a manages the various packets that are stored in the buffer 21. Whendata is taken over in a handover, the control unit 24 forwards at leastthe packets that are stored in the buffer unit 21 b for which aconfirmation (ACK) indicating that the packets were rightly received hasnot been obtained from the mobile station to the target base station 11b.

On the other hand, when the arrival of packets that are being forwardedfrom the source base station 11 a are delayed due to the take over ofdata, and jump transmission is performed for the packets that arereceived from the host station 12, ‘type=010’ is entered into the typefield of the headers of the packets that will jump.

The HO control unit 24 b executes handover control as explained in FIG.23, and the measurement control unit 24 c gathers various measurementdata that is sent from the mobile station, such as the radiocommunication quality CQI (Channel Quality Information) of the mobilestation and the like.

Construction of a Mobile Station

FIG. 6 is a drawing showing the construction of a mobile station, andshows the transmission/reception unit 31, buffer unit 32, reorderingunit 33 and control unit 34. The transmission/reception unit 31transmits packets and control information to or receives packets andcontrol information from a base station. When RLC PDU data could not becreated from a lower-layer packet that was received, the buffer unit 32holds that lower-layer packet until that RLC PDU data can be created,and after the RLC PDU data is created, the buffer unit 32 removes theheader and delivers that data to the reordering unit 33 as RLC SDU (PDCPPDU) data. The reordering unit 33 has a function for rearranging thePDCP PDU data in order of sequence numbers and delivers the data to theupper layer. When a missing PDCP PDU sequence number is detected, thereordering unit 33 saves the PDCP PDU data following that PDCP PDU datain internal memory until the PDCP PDU data with the continuing sequencenumber is received. However, when that PDC PDU data has still notarrived after a set period of time has elapsed, the reordering unit 33stops the reordering process and delivers all of the stored PDCP PDUdata to the upper layer. Moreover, the reordering unit 33 performswindow control so that the amount of data being processed does notexceed the allowable amount.

The control unit 34 comprises a measurement unit 34 a, reorderingmanagement unit 34 b and retransmission management unit 34 c. Themeasurement unit 34 a measures various kinds of measurement informationthat are sent to a base station. For example, the measurement unit 34 ameasures the radio communication quality (Channel Quality Information)of the mobile station. The reordering management unit 34 b controls thereordering unit 33, and when there is a missing sequence number in thePDCP PDU data that is held by the reordering unit 33, the reorderingmanagement unit 34 b instructs the reordering unit 33 to wait for thatPDCP PDU data having the continuing sequence number to arrive. Moreover,when a set amount of time for waiting for that packet to arrive haselapsed, the reordering management unit 34 b instructs the reorderingunit 33 to stop the reordering process, as well as instructs thereordering unit 33 to delete the headers of all of the saved PDCP PDUdata and to delivers that data to the upper layer as PDCP SDU data, thensets the reordering unit 33 to a state in which it is able to receivenew PDCP PDU data. In addition, the reordering management unit 34 bfinds the maximum sequence number from among sequence numbers that havebeen received up to that time as the sequence number on the right end ofthe window, and determines and sets the sequence number of the left endof the window by taking into considering the window size. Here, in thecase where there is a packet that has already been received that has asequence number that is smaller than the sequence number on the left endof the window, that packet is immediately delivered to the upper layer.When there is retransmission control, the retransmission management unit34 c sends a retransmission request signal to a base station via thetransmission/reception unit by a route indicated by the dashed line.

Operation of the Target Base Station

FIG. 7 is a flowchart showing the operation of the target base stationin a first embodiment of the invention.

When the handover control unit 24 b of the target base station 11 breceives a HO request from the source base station 11 a (including themobile station ID, QoS information, and the like) (step S201), thehandover control unit 24 b determines whether to allow accepting themobile station (step 202). When accepting the mobile station is notallowed, the handover control unit 24 b performs post processing (step213) and ends handover control.

On the other hand, in a case where the handover control unit 24 b allowsthe acceptance of the mobile station, the handover control unit 24 breturn a HO request response message to the source base station 11 a(step 203). Then the target base station waits the packets that areforwarded from the source base station 11 a. When the packets arriversfrom the source base station, the buffer 21 stores them (step 204).

When the handover control unit 24 b receives a HO complete report fromthe mobile station 14 (step 205), the handover control unit 24 b thensends a HO complete report to the host station 12 (step 206). Afterreceiving the HO complete report, the host station 12 changes thetransmission path for packets from the source base station 11 a to thetarget base station 11 b, and returns a HO complete response to thetarget base station 11 b. After the handover control unit 24 b of thetarget base station 11 b receives the HO complete response from the hoststation 12 (step 207), the handover control unit 24 b sends aninstruction to the scheduler unit 22 to start transmitting packets (step208).

From this, the scheduler unit 22 checks whether jump transmission ofpackets is necessary (step 209), and when jump transmission is notnecessary, the scheduler unit 22 sends the packets that were forwardedfrom the source base station 11 a to the mobile station 14 first (step210). However, when forwarding of packets from the source base station11 a is delayed, and it is necessary to perform jump transmission ofpackets, the scheduler unit 22 executes jump transmission of the packetsthat were received from the host station 12.

In order to execute jump transmission, the scheduler unit 22 sets thetype number in the type fields of the packets for which jumptransmission will be performed to 010 (type=010) in order that themobile station 14 will be able to recognize that the packets are jumppackets (step 211), then after that, sends the packets to the mobilestation 14 (step 210).

At the same time as this, the handover control unit 24 b sends aresource release to the source base station 11 a (step 212), thenperforms post processing (step 213) and ends handover control.

Operation of the Source Base Station

FIG. 8 is a flowchart showing the operation of a source base station ina first embodiment of the invention.

In FIG. 8, when the measurement control unit 24 c of the source basestation 11 a receives a Measurement Report which indicates receptionstate information from the mobile station 14 (step 251), the handovercontrol unit 24 a determines whether or not a handover (HO) is necessarybased on that reception state information (step 252), and when ahandover is not necessary, returns to the start.

However, when the handover control unit 24 b determines that a handoverHO is necessary, the handover control unit 24 b decides a target basestation 11 b according to the contents of the Measurement Report andsends a handover request to that target base station 11 b (step 253).

After that, when a HO response message that is sent from the target basestation 11 b is received (step 254), the HO control unit 24 b sends a HOinstruction message to the mobile station 14 (step 255), and instructsthe buffer management unit 24 a to forward packets that are saved in thebuffer 21 b to the target base station 11 b. Thereby, using the routeindicated by the dotted line, the buffer management unit 24 a forwardspackets that are saved in the buffer 21 b and that were not sent to themobile station 14, or packets that were not received rightly (NACKpackets) by the mobile station, to the target base station 11 b (step256). After that, when a resource release message is received from thetarget base station 11 b (step 257), the HO control unit 24 a performs aresource release (step 258).

Operation of the Mobile Station

FIG. 9 is a flowchart showing the operation of the mobile station. Themeasurement unit 34 a of the mobile station 14 notifies the source basestation 11 a of the reception status using a Measurement Report (step271). The control unit 34 then waits for a HO instruction message to besent from the source base station 11 a, and when a HO instructionmessage is received (step 272), the control unit 24 establishessynchronization between the mobile station and the target base station11 b using L1/L2 signaling (step 273), and after synchronization isestablished, sends a handover complete report to the target base station11 b (step 274). After that, the control unit 34 checks whether areceived packet is a jump packet, or in other words whether the typenumber of a packet is type=010 (step 275), and when a packet is a jumppacket, the control unit 34 removes that packet as an object ofreordering and saves the packet in a buffer 32, then after a set periodof time has elapsed, removes the header and delivers the packet to theupper layer as a PDCP SDU packet (step 276). On the other hand, when apacket is not a jump packet, the control unit 24 executes reordering,and rearranges the order according to the sequence number, then removesto header from the reordered packets and delivers them to the upperlayer as a PDCP SDU packets (step 277).

FIG. 10 is a flowchart showing the reordering process by the mobilestation.

When the transmission/reception unit 31 of the mobile station 31receives a lower layer packet from the target base station 11 b (step301), the reordering management unit 34 b checks whether it is possibleto create RLC PDU data (step 302), and when it is not possible to createRLC PDU data, checks whether a set amount of time has elapsed (step303), and when the set amount of time has not elapsed, saves the lowerlayer packet in the buffer 32 (step 304), then performs processing againfrom step 301. When it is not possible to create RLC PDU data eventhough a set amount of time has elapsed since the lower layer packet wasreceived, that lower layer packet is deleted from the buffer (step 305).

On the other hand, in step 302, when it is possible to create RLC PDUdata using the received lower layer packet, that RLC PDU data isdelivered to the reordering unit 33 as RLC SDU (PDCP PDU) data (step306). After receiving the RLC SDU (PDCP PDU) data, the reordering unit33 checks whether there is a missing sequence number (step 307), andwhen there is no missing sequence number and the sequence numbers arecontinuous, the reordering unit 33 removes the header of that RLC SDU(PDCP PDU) data and delivers it to the upper layer as PDCP SDU data(step 311). However, when there is a missing sequence number, thereordering management unit 34 b instructs the reordering unit 33 to savethe PDCP PDU data (step 308). Thereby, the reordering unit 33 saves theRLC SDU (PDCP PDU) data, and checks whether RLC SDU (PDCP PDU) data witha continuing sequence number is received (step 309). After receiving RLCSDU (PDCP PDU) data with a continuing sequence number, the reorderingunit 33 removes the header of the RLC SDU (PDCP PDU) data and deliversit to the upper layer as PDCP SDU data, and then delivers similarly thesaved PDCP PDU as PDCP SDU to the upper layer (step 311).

Moreover, in step 309, when RLC SDU (PDCP PDU) data having a continuingsequence number is not received, the reordering unit 33 monitors apreset period of time T_(M) (step 310), and when the set period of timehas not yet elapsed, repeats processing from step 308, however, when theset period of time T_(M) has elapsed, the reordering unit 33 removes theheader of the saved PDCP PDU data and delivers it to the upper layereven though the sequence numbers are not continuous (step 311).

With the first embodiment of the invention as described above, packetsthat are transmitted from the host station 12 to the target base station11 b can be transmitted to the mobile station 14 as jump packets withoutwaiting, so it is possible to eliminate delay time of the data, and toimprove the throughput of the overall system. Moreover, the mobilestation 14 removes jump packets as objects of reordering control(sequence order control), and performs sequence order control on packetsother than jump packets and delivers those packets to the upper layer inorder of sequence numbers. As a result, the mobile station is able toperform sequence order control of packets even when there is only oneorder control function.

(C) Second Embodiment

In the first embodiment, the mobile station 14 performed the reorderingprocess during a set period of time TM, and ended the reordering processwhen that set period of time TM elapsed (see step 310 in FIG. 10). Inthat case, when the set period of time TM has not yet elapsed eventhough all of the packets that have been forwarded from the source basestation 11 a to the target base station 11 b are received, the mobilestation 14 continues the reordering process. Therefore, in a secondembodiment of the invention, in order that the mobile station 14 is ableto recognize the last packet that is an object of the reorderingprocess, the target base station 11 b adds an identifier to a specifiedpacket (last packet) and transmits that packet to the mobile station 14,and after receiving that last packet, the mobile station 14 immediatelyends the reordering process even though the set time TM has not elapsed.

FIG. 11 is a drawing explaining a second embodiment of the invention,where it presumed that packets n−5 to n are stored at the source basestation 11 a before a handover, and of these packets, packets n−5 to n−2were sent to the mobile station 14, however, packets n−1 and n were notsent to the mobile station 14. For example, packets n−1 and n arrivedafter the radio communication line between the source base station 11 aand the mobile station had been cut, so these packets n−1, n were notsent to the mobile station 14. In addition, it is presumed that of thepackets that were sent to the mobile station 14, packets n−5 and n−3were not received rightly by the mobile station 14 (NACK), and thatpackets n−4 and n−2 were received rightly (ACK). Therefore, the mobilestation 14 saves packets n−4 and n−2, and does not save packets n−5 andn−3.

When a handover occurs in this state, the source base station 11 aforwards the packets n−5 and n−3 that were not rightly received by themobile station 14, and the unsent packets n−1 and n to the target basestation 11 b. Also, the host station 12 sends two packets m to m+1 tothe target base station 11 b after the handover. It is presumed that theforwarding of the packets n−5, n−3 and n−1 to n is delayed.

When the target base station 11 b receives the packets m and m+1 fromthe host station 12 before the packets n−5, n−3 and n−1 to n areforwarded from the source base station 11 a, the target base station 11b adds jump ID code F to the packets m and m+1 that were received fromthe host station and sends them to the mobile station 14 first (jumptransmission). The mobile station 14 saves the packets received from thebase station with the jump ID code added in the buffer BF2, and removesthem as objects of the reordering process. In (A) of FIG. 11, the stateis shown in which the mobile station 14 saves packets m and m+1 in thebuffer BF2, and saves the packets n−4 and n−2 that were received beforethe handover in the buffer BF1.

The target base station 11 b sends the packets that are forwarded untila preset time T_(M) (called the Waiting Time) has elapsed to the mobilestation 14, and when the Waiting Time has elapsed the target basestation 11 b discards any packets that may be forwarded. Therefore, thetarget base station 11 b receives the packets n−5, n−3 and n−1 that wereforwarded from the source base station 11 a before the Waiting Timeelapsed and saves them in the buffer BF, then sends the packets one byone to the mobile station 14. In addition, the target base station 11 breceives packet n from the source base station 11 a, however, theWaiting Time was complete before the packet n could be sent to themobile station 14. In this case, the target base station 11 b adds anidentifier (L) to packet n to indicate that it is the last packetforwarded from the source base station, and sends packet n to the mobilestation 14.

The mobile station 14 saves the packets n−5, n−3, n−1 and n that werereceived from the target base station 11 b in the buffer BF1, and asshown in (B) of FIG. 11, executes the reordering process to rearrangethese packets and the packets n−4 and n−2 that were received beforehandover. Moreover, after detecting packet n with the identifier (L)added, the mobile station 14 determines that forwarding has ended, sodelivers the packets that have been reordered to the upper layer andends the reordering process even though the set time period T_(M) forreordering has not ended.

The case in which the Waiting Time T_(w) ends before packet n is sent isdescribed above, however, the Waiting Time T_(w) may end after thepacket n has been sent to the mobile station. In that case, the targetbase station 11 b adds an identifier (L) to a packet m+2 that will bereceived from the host station 12 indicating that it is the last packet,and sends the packet m+2 to the mobile station 14. After receiving thelast packet to which that identifier (L) has been added, the mobilestation 14 determines that forwarding has ended, and immediately endsreordering even though the set period of time T_(M) for reordering hasnot ended.

Moreover, packets n−5 and n−3 are received by forwarding, however, theWaiting Time T_(w) may end in the stage before packets n−1 and n arereceived. In that case, the target base station 11 b may add anidentifier (L) to packet n−3 indicating that it is the last packet, andsends that packet to the mobile station 14. The mobile station 14detects the Last packet to which that identifier (L) is added and endsthe reordering process.

Last Packet ID Code L

To add an identifier (last packet ID code) L to a packet identifyingthat the packet is the last packet, a 3-bit ‘type’ field that isincluded in the PDCP PDU header is used. In other words, a new typenumber is defined in that ‘type’ field as the last packet ID code L, andthat type number is assigned to the first packet to be sent after theWaiting Time T_(w) ends. FIG. 12 shows an example of the PDCP PDUformat, where (A) is an example of format without a header, (B) is anexample of format in which the PDCP PDU sequence number is not added,and (C) and (D) are examples of format in which the PDCP PDU sequencenumber is added. In the format shown in (C) and (D), a type field and aPID field are defined in the header HD, where the type field indicatesthe type of PDCP PDU. For the type field, ‘type=000’ and ‘type=001’ arealready regulated, however, the type numbers ‘type=010 to 111’ are notregulated and are unused. Therefore, ‘type=011’ is used as the typenumber for recognizing the PDCP PDU packet (last packet) that is to besent first after the Waiting Time T_(w) has ended.

PDCP Layer and RLC Layer Processing

FIG. 13 and FIG. 14 are drawings explaining packet processing for thePDCP layer and RLC layer after a handover.

FIG. 13 shows the case in which the last packet ID code L is added tothe packet n that is sent to the mobile station 14. In the firstembodiment, during reordering, the mobile station 14 starts a timer inorder to determine the end of reordering. Here, when the set time T_(M)is set to be a large value, the reordering process continues regardlessof whether or not packet n is the last packet forwarded, and the packetscannot be delivered to the upper layer until the set time T_(M) ends.However, in this second embodiment, after receiving packet n (Lastpacket) to which the last packet ID code L has been added, the mobilestation 14 immediately ends reordering and delivers all of the PDCP PDUpackets to the upper layer.

When more than the allowable amount of data arrives at the mobilestation 14, the mobile station 14 performs window control duringreordering. The left side of the window is n−5, which is the numberexpected for arrival, and the right side of the window is the value ofthe upper limit of the allowable amount of data (in the figure, thisvalue is n). However, window processing is not applied to ‘type=010’packets (jump packets) as in the first embodiment. By performingprocessing in this way, it is possible to execute reordering from packetn−5 to packet n.

FIG. 14 shows the case in which last packet ID code L is added to packetm+2. At the instant that packet m+2 (Last packet) is received, themobile station 14 immediately ends reordering even though the set timeT_(M) may not have ended yet, and delivers all of the received PDCP PDUpackets to the upper layer. Also, in the case of window control, themobile station 14 executes window control in the same way as shown inFIG. 13.

Operation of the Target Base Station

FIG. 15 is a flowchart showing the operation of the target base stationin the second embodiment of the invention, where the same referencenumbers are given to steps that are the same as the steps of the firstembodiment shown in FIG. 7. The base stations and mobile station of thissecond embodiment have the construction shown in FIG. 5 and FIG. 6.

When the handover control unit 24 b of the target base station 11 breceives a HO request from the source base station 11 a (includes themobile station ID, QoS information, etc.), the handover control unit 24b performs call-receiving control based on that information anddetermines whether to allow acceptance of the mobile station. Whenacceptance is not allowed, the handover control unit 24 b performs postprocessing, and ends handover control (steps 201 to 202, and 213).

On the other hand, when the handover control unit 24 b allows theacceptance of the mobile station 14, the handover control unit 24 breturns a HO request response message to the source base station 11 a,and starts measuring the elapsed time. After that, the target basestation 11 b waits for packets forwarded from the source base station 11a, and stores the packets in the buffer unit 21 (steps 203 and 204).

In this state, after receiving a HO complete report from the mobilestation 14, the handover control unit 24 b sends a HO complete report tothe host station 12 (steps 205 and 206). After the host station 12receives the handover complete report, the host station 12 then changesthe transmission path for the packets from the source base station 11 ato the target base station 11 b, and returns a HO complete response tothe target base station 11 b. The handover control unit 24 b of thetarget base station 11 b receives the HO complete response from the hoststation 12, and then instructs the scheduler unit 22 to starttransmitting packets to the mobile station(steps 207 and 208).

Next, the scheduler unit 22 monitors whether or not the elapsed time hasexceeded a set period of time T_(w), that is, whether or not the WaitingTime has ended (step 501), and when the Waiting Time has not ended,checks whether it is necessary to perform jump transmission of packets(step 209), and when it is not necessary, starts sending packets thatwere forwarded from the source base station 11 a (step 210)preferentially to the mobile unit 14 via transmission/reception unit 23.However, when forwarding is delayed and it is necessary to perform jumptransmission of packets, the scheduler unit 22 executes jumptransmission of packets that are received from the host station 12. Inorder to execute jump transmission, the scheduler unit 22 sets the typenumber of the type field in the header of the packets for which jumptransmission will be performed to 010 (type=010), thereby the mobilestation 14 will be able to identify that the packets are jump packets(step 211). After that, the transmission/reception unit 23 sends thejump packets to the mobile station.

At the same time as this, the handover control unit 24 b sends aresource release to the source base station 11 a (step 212), thenreturns to step 501 to check whether the Waiting Time has ended, andwhen the Waiting Time has not ended, repeats the process from step 209.

On the other hand, in step 501, when the Waiting Time T_(w) has ended,the scheduler unit 22 sets the type number of the packet that will betransmitted immediately after the Waiting Time has ended to ‘011’. Inother words, the scheduler unit 22 adds a last packet ID code L to thepacket that will be transmitted immediately after the Waiting Time ends,and transmits that packet (Last packet) to the mobile station 14 (steps502 and 503), then after that, performs post processing and endshandover control (steps 212 and 213).

In step 502, in order to be able to identify whether the last packet towhich the last packet ID code L has been added is a packet that has beenforwarded from the source base station 11 a, or is a packet that wasreceived from the host station 12, the scheduler unit 22 sets the typenumber to ‘011’ in the case of the former, and sets the type number to‘100’ in the case of the latter. By doing this, it becomes easy for themobile station 14 to perform the reordering process as will be describedlater.

Operation of the Mobile Station

FIG. 16 is a flowchart showing the operation of the mobile station,where the same reference numbers are used for steps that are the samethose in the operation flowchart shown in FIG. 98. The measurement unit34 a of the mobile station 14 uses a Measurement Report to notify thesource base station 11 a of the reception status. The control unit 34then waits for a HO instruction message to be sent from the source basestation 11 a, and after receiving the HO instruction message,establishes synchronization between the mobile station 14 and the targetbase station 11 b using L1/L2 signaling, and after synchronization hasbeen established, sends a handover complete report to the target basestation 11 b (steps 271 to 274).

After that, the control unit 34 checks whether a received packet is ajump packet, or in other words, a packet having a type number type=010(step 600), and when the packet is a jump packet, removes that packetfrom being an object of reordering and saves it in the buffer 32 (step601).

On the other hand, when the packet is not a jump packet, the controlunit 34 checks whether the received packet is the Last packet, or inother words, a packet having a type number type=011 (step 602). In thecase where the packet is not the Last packet, the control unit 34 startsthe reordering process, and arranges the PDCP PDU packets according tosequence numbers, then creates PDCP SDU data and delivers that data tothe upper layer (step 603).

The control unit 34 then checks whether the preset time period T_(M) hasended (step 604), and when the set time period T_(M) has not yet ended,the control unit 34 repeats processing from step 600, and when the settime period T_(M) has ended, the control unit 34 ends the reorderingprocess, then deletes the header from RLC SEU (PDCP PDU) packets thathave not yet been delivered to the upper layer, creates PDCP SDU dataand delivers that data to the upper layer (step 605). Next, the controlunit 34 delivers packets that are not the object of reordering to theupper layer and ends processing (step 606).

However, in step 602, when the received packet is the Last packet, thecontrol unit 34 immediately stops the reordering process (step 607). Atthat time, in the case where the Last packet is a forwarded packet, thecontrol unit 34 creates a PDCP SDU packet from that Last packet anddelivers that packet to the upper layer, then stops reordering. When theLast packet is a packet received from the host station 12, the controlunit 34 immediately stops reordering, and connects the Last packet tothe last packet that is not an object of reordering and stored in thebuffer. After that, the control unit 34 delivers the packets that werenot the object of reordering to the upper layer and ends processing(step 606).

As described above, with this second embodiment of the invention, themobile station detects the end of the forwarded packets by referencingthe Last packet ID code, making it possible to quickly stop thereordering process. Therefore, it is possible to eliminate the datadelay time, and improve the throughput of the overall system.

(D) Third Embodiment

The order information (sequence numbers n, m) that has been presented upuntil now has been convenient numbers for simplifying the explanation,however, actually, it is information that is added at the base station.

As was explained above, in a LTE communication system, forwarding isperformed in PDCP SDU data units. Therefore, when forwarding occurred,it was not possible to send the Sequence Number field from the sourcebase station 11 a to the target base station 11 b, and it was necessaryto notify the target base station 11 b of the sequence numbers usingsome method.

In this embodiment, the source base station 11 a notifies the targetbase station 11 b of the sequence numbers together with the PDCP SDUdata, and based on those sequence numbers, the target base station 11 badds a header including the sequence numbers to the forwarded PDCP SDUdata (packets) thereby PDCP PDU(RLC SDU) packet is created.

FIG. 17 is an example of sending the PDCP PDU sequence number togetherwith the PDCP SDU data (packets) via the data plane (U-plane). In FIG.17, only the PDCP SDU data and the sequence numbers that accompany thatdata are shown, however, in order that the PDCP SDU data is recognizedto be a bunch of packets, control information (header information) mustbe added. Each time the source base station 11 a forwards PDCP SDU data,the source base station 11 a notifies the target base station 11 b ofthe sequence number that accompanies that PDCP SDU data using the formatshown in FIG. 17, and based on that sequence number, the target basestation 11 b adds the sequence number to the forwarded PDCP SDU data(packet).

In other words, in the condition shown in FIG. 17, the source basestation 11 a first forwards sequence number n−5 and the first PDCP SDUdata via the data plane U-plane. Next, the source base station 11 aforwards sequence number n−3 and the next PDCP SDU data via the dataplane U-plane. After that, when forwarding following PDCP SDU data thatwere received from the host station 12, the source base station 11 asimilarly forwards the PDCP SDU data and the sequence numbers n−1 and nthereof to the target base station 11 b via the U-plane. At the targetbase station 11 b, by receiving the sequence numbers n−5, n−3, n−1 and nthat were notified via the U-plane, it is possible for the target basestation 11 b to recognize the numbers as the sequence numbers for theforwarded data.

FIG. 18 is an example of the source base station 11 a notifying thetarget base station 11 b via the U-plane of the PDCP PDU sequence numbertogether with the appropriate PDCP SDU data, and notifying the targetbase station 11 b via the C-plane of the sequence number of PDCP SDUdata, for which right reception by the mobile station could not beconfirmed, as the Next SN. Notification of the sequence number (Next SN)via this C-plane is shown as taking over of the SN (SN takeover) in thehandover sequence shown in FIG. 19. It may possible to perform SN takingover at the same time that the source base station 11 a sends a HOrequest to the target base station 11 b.

In the state shown in FIG. 18, the source base station 11 a firstforwards the sequence number n−5 and the first PDCP SDU data via theU-plane. Also, at the same time, the source base station 11 a notifiesthe target base station via the C-plane of the just the sequence numbern−5 of the PDCP SDU data, for which proper reception by the mobilestation could not be confirmed, as the Next SN (SN takeover). Next, thesource base station 11 a forwards the sequence number n−3 and the nextPDCP SDU data via the U-plane. After that, when forwarding the PDCP SDUdata that was received from the host station 12, the source base station11 a similarly forwards that PDCP SDU data and the sequence number n−1,n thereof to the target base station 11 b via the U-plane. The targetbase station 11 b receives the sequence number n−5 that was notified viathe C-plane, and the sequence numbers n−5, n−3, n−1 and n that werenotified via the U-plane, however, since the sequence numbers that werenotified via the U-plane are larger than the sequence number notifiedvia the C-plane, the target base station 11 b adds the sequence numbersthat were notified via the U-plane to the following PDCP SDU datareceived via the U-plane, and transmits the data to the mobile station.In other words, the target base station 11 b ignores the sequence numbern−5 that was notified via the C-plane.

FIG. 20 is an example of the case in which there is no PDCP SDU data toforward.

The source base station 11 a notifies the target base station 11 b viathe C-plane of the next sequence number n+1, as the Next SN. When theWaiting Time ends without the target base station 11 b receiving PDCPSDU data from the source base station 11 a, the target base station 11 badds the sequence number to the following PDCP SDU data based on thesequence number n+1 that was notified via the C-plane, and transmits thedata to the mobile station. In other words, the source base station 11 bsets the sequence number of the packet m that was received from the hoststation to n+1 and transmits that data to the mobile station.

Advantage of the Present Invention

With the present invention described above, it is possible to quicklysend packets that were sent from host station to the target base stationto the mobile station as jump packets, thus making it possible toeliminate the delay time of data, and to improve the throughput of theoverall system. Moreover, the mobile station removes jump packets frombeing the object of reordering control (sequence order control), andperforms order sequence control only on packets other than jump packets,then delivers the packets to a higher apparatus (upper layer) in orderof sequence numbers. As a result, the mobile station is able to properlyperform sequence order control of packets that are the object ofreordering even when there is only one sequence order function.

Furthermore, with the present invention, the mobile station detects theend of the forwarded packets by referencing last packet ID code, and sois able to quickly stop reordering. Therefore, it is possible toeliminate the delay time of data, and to improve the throughput of theoverall system.

What is claimed is:
 1. A radio communication system, comprising: a source base station; a target base station; and a mobile station for receiving either first sequence number or second sequence number, or both of the first and second sequence numbers from the target base station, wherein the first sequence number is transferred from the source base station to the target base station and the second sequence number is added to PDCP (Packet Data Convergence Protocol) SDU (Service Data Unit) which is transferred from the source base station to the target base station.
 2. The radio communication system according to claim 1, further comprising: means for transferring the first sequence number via a control plane from the source base station to the target base station; and means for transferring the PDCP SDU on which the second sequence number is attendant via a user plane from the source base station to the target base station.
 3. A mobile station comprising: means for receiving either first sequence number or second sequence number, or both of the first and second sequence numbers from a target base station, wherein the first sequence number is transferred from a source base station to the target base station and the second sequence number is added to PDCP (Packet Data Convergence Protocol) SDU (Service Data Unit) which is transferred from the source base station to the target base station.
 4. A base station, comprising: means for receiving first sequence number and PDCP (Packet Data Convergence Protocol) SDU (Service Data Unit) from a source base station; means for transmitting either the first sequence number or second sequence number which is added to the PDCP SDU, or both of the first and second sequence numbers, to a mobile station. 